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July 2008

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Subject:
From:
"Stadem, Richard D." <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Stadem, Richard D.
Date:
Wed, 16 Jul 2008 08:34:03 -0500
Content-Type:
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Hi, Keith
Thank you for your input on this. 
It raises five questions:

1. Does the elimination of such a small amount of silver from the alloy
contribute to any significant increase in the solder's ductility after
soldering is completed?  

2.Specifically, can you provide data as to exactly what the ductility
delta is between all three alloys? I would expect a significant
difference between 63/37 and SN100 or SAC305, due to the percentage of
lead.

3. Also, with the understanding that VPS would probably provide better
wetting, how would it otherwise affect the ductility of the alloy after
soldering is completed? The profile used is essentially the same.

4. For a given assembly that has inherently high CTE stresses due to
design issues as Amol's assembly presumably has, are you saying that
using SN100 and VPS rather than SAC305 and convection reflow will cure
his problems? 

5. How do the long-term creep fatigue rates of Sn100 compare with SAC305
and SN63Pb37?

I am not a metallurgist, and I am at a loss as to where to find this
information.

-----Original Message-----
From: Leadfree [mailto:[log in to unmask]] On Behalf Of [log in to unmask]
Sent: Wednesday, July 16, 2008 5:03 AM
To: [log in to unmask]
Subject: Re: [LF] [TN] SN100 for Reflow Application

Although the correspondence under this title has been very interesting
and informative this string drifted somewhat from the original question,
which was:

From: "Kane, Amol (349)" <[log in to unmask]>
Subject: SN100 for Reflow Application
X-To: TechNet E-Mail Forum <[log in to unmask]>, Dear technetters, Is
anybody using SN100 (or other high Sn alloys) for SMT reflow soldering
applications? what have your experiences been with this alloy?

I have a PCB with a BGA, that is cracking due to mechanical stress (that
is not SMT process related), we have a combination of solder cracking,
and pad cratering. The PCB currently uses SAC305. We also build a SnPb
variant of the same exact board with no issues (the bare board material
is different for SnPb and LF versions). I was therefore wondering
whether SCA305 alloy properties could be a contributing factor this
defect. How does the ductility and shear strength of SN100C compare to
SAC305 and 67-37SNPb alloy?

To return to the original question, since it has not been clear from the
correspondence exactly what the nature of the crack was I cannot say
with any 
certainty whether the result would be any better with SN100C than with
SAC305.   
However, to answer the specific question on the ductility and shear
strength of SN100C, it is a more ductile alloy than SAC305 and has a
lower flow stress 
and in that regard is more like 63/37 SnPb.   If the cracking occurred
in a 
situation where the joint was subjected to substantial strain then the
greater compliance of the SN100C could mean that the solder would
accommodate that strain without cracking and without transmitting the
stress to the substrate or the component.  Cratering can be a symptom of
unrelieved stress being transmitted into the laminate instead of being
absorbed by deformation of the solder itself, e.g. the cratering failure
with area array devices in dropped cell phones 
with SAC305 joints.   Cracking of chip capacitors can be another
consequence of 
strain being transmitted rather than accommodated by the solder.

In regard to a later comment on the poor results obtained in reflow with
the 99C alloy (Sn-0.7Cu), although SN100C is based on that same alloy
microalloying with Ni and Ge substantially changes the behaviour of the
alloy, as has been well documented in several peer-reviewed scientific
papers.  Reflowed SN100C joints are smooth, bright and generally free of
shrinkage defects and can be difficult to distinguish from 63/37SnPb
joints. 

The higher melting point of SN100C (227C/440F compared with the
215-220C/419-428F of SAC305) has been raised as a concern but the
experience of the increasing number of lines running with SN100C paste
indicates that because of the modifying effect of the microalloying
additions the alloy needs less superheat 
(excess of the peak temperature over the melting point) than SAC305.
For 
reflow lines reflowing SAC305 with a peak temperature around
240-245C/464-473F, which seems to be a profile very commonly used with
that alloy, SN100C has 
proved to a "drop-in-replacement".   However, depending on the Delta T,
we would be 
more cautious about suggesting SN100C would be a drop-in-replacement for

SAC305 for lines running with a peak temperature 230-235C/446-455F.
However, 
results obtained with the new generation of vapour phase reflow systems
indicate that good results can be achieved with SN100C with a liquid
with a boiling point of 235C/455F.

Keith Sweatman
Nihon Superior Co., Ltd
    
    
    



    
    
    





   

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